Kainite mixed salt is obtained from salt bitterns. In pure form it is a double salt of KCl.MgSO4.3H2O although as obtained from salt bitterns it typically contains impurities of NaCl and MgCl2.6H2O.
Potassium sulphate is a dual fertilizer containing 50% K2O and 18% Sulfur. It has other applications also as documented in the prior art.
Magnesium hydroxide is commercially used in pulp and paper industries and also as antacid, fire retardant, mild base and as intermediate in the production of fertilizer, magnesia and diverse magnesium chemicals.
Ammonium sulphate is used as dual fertilizer containing 21.5% N and 24.6% S. It is also used as raw material for production of various chemicals.
Reference may be made to U.S. Pat. No. 7,041,268, May 9, 2006 by Ghosh et al. which covers extensively the prior art related to production of potassium sulphate (also referred to as sulphate of potash or SOP) from bittern sources.
Reference may again be made to the same patent which discloses the preparation of potassium sulphate and magnesia in integrated manner from kainite mixed salt and lime. Gypsum is obtained as a by-product. Crude kainite salt obtained from salt bitterns through solar evaporation is treated with water and a waste stream of the process to convert it into schoenite (K2SO4.MgSO4.6H2O) while simultaneously leaching out unwanted impurities present in the crude kainite. The leachate, containing K+, Na+, Mg2+, Cl− and SO42− is desulphated with CaCl2 and allowed to further evaporate to recover carnallite double salt (KCl.MgCl2.6H2O) which is decomposed with water and hot leached to obtain KCl in pure form while the Mg2+-rich mother liquor is treated with lime to produce Mg(OH)2 with co-production of CaCl2 required for desulphatation. Schoenite and KCl are treated with water to recover K2SO4 in solid form while the mother liquor is recycled in the kainite decomposition step. The main drawback of the process is that the quality of lime can vary and, with it, the quality of the magnesium hydroxide. Another drawback is the disposal of by-product gypsum for which it is difficult sometimes to find a market.
Reference may be made to U.S. Pat. No. 4,504,458, Mar. 12, 1985 by Knudsen which discloses a process for converting gypsum into potassium sulphate or sodium sulphate by subjecting aqueous slurry of gypsum with anion exchanger (in chloride form) under acidic conditions which resulted into calcium chloride solution and resin in sulfate form. The anion exchange resin is then contacted with potassium chloride or sodium chloride solution thereby regenerating the sulphate loaded resin to form chloride loaded resin and potassium sulfate or sodium sulfate solution.
Reference may be made to E. Sacher, ISMA Tech. Conf. 1968 which describes the Merseberg process for the manufacture of ammonium sulfate from natural gypsum.
Reference may also be made to the article “Disposal or use of gypsum in production of ammonium sulfate” by N. D. Gopinath in Phosphoric Acid, Vol. 1, Part II, (ed. A. V. Slack), Marcel Dekker, New York, 541-566 (1968) which discusses the conversion of gypsum into ammonium sulfate by its reaction with ammonium carbonate. No detailed process parameters are given or are any mention made of the utility of the by-product calcium carbonate.
During the wet process manufacture of phosphoric acid, the basic raw material from which most phosphatic fertilizers are made, concentrated phosphate rock is reacted with sulfuric acid which results in the production of substantial quantities of by-product gypsum known as “phosphogypsum”. Various proposals have been made for converting phosphogypsum and gypsum into useful and economical products by chemical means. In every instance, although technically feasible, the cost of the chemicals required to carry out the conversion has been greater than the value of the resulting product. An example is the reaction of gypsum with ammonia and carbon dioxide to form ammonium sulfate and calcium carbonate. Because of its low purity as compared to natural gypsum, the use of phosphogypsum has not proven economical in this manner.
Reference may be made to the article by M. Chou et al. in www.anl.gov/PCS/acsfuel/preprint%20archive/Files/40—4_CHICAGO—08-95—0896.pdf
This teaches the production of ammonium sulfate and calcium carbonate from gypsum. The reaction produces insoluble calcium carbonate and an ammonium sulfate solution. The authors state therein that the technique, although attractive in principal, is not commercially viable due to issues around cost and quality of natural gypsum and the lack of availability of an economical source of carbon dioxide.
Reference may be made to the Solvay Process which teaches the preparation of soda ash from NaCl, NH3 and CO2 with co-generation of NH4Cl. CO2 is obtained by calcination of limestone (CaCO3) while the lime obtained is reacted with NH4Cl to recycle the ammonia with co-production of waste CaCl2 which is let out as effluent.
Magnesium oxide is an important compound that finds application in various industries. Magnesium oxide has the highest melting point of the moderately priced oxides and is therefore an important raw material for refractory bricks and other materials. It is the only material apart from ZrO2 that can withstand long term heating above 2000° C.
Reference may be made to US Patent application 20007019121 by Ghosh et al. which discloses a process for the preparation of MgO from the reaction of magnesium salt with caustic soda or lime. The crude Mg(OH)2 is directly calcined and then treated with water to disintegrate the mass spontaneously to yield a slurry and dissolve away the soluble salts. This slurry is much easier to filter and wash than the original Mg(OH)2 slurry. No mention is made of any process to speed up filtration of the Mg(OH)2 slurry itself.